(1) Field of the Invention
The invention relates to a package structure and method for preventing gold bonding wires from collapsing, which is especially useful for those chips whose arrays of bonding pads are on the chip center to be packaged on a ball grid array (BGA) substrate.
(2) Description of the Prior Art
In order to meet higher and higher density integrated circuit on a chip, the number of I/O pad must increase synchronously. The pad pitch is smaller than that in the past due to limited area, so that it is more difficult to use the conventional packaging technology. Therefore, manufacturers of package must find new ways of doing things from old packaging technology for the requirements of customers. Even so, the manufacturers still face a lot of difficulties, for example, the gold bonding wire is excessively long if a BGA is fabricated on the chip which has arrays of bonding pads on the chip center.
Referring to FIG. 1A and FIG. 1B, a solder ball 5 is disposed under the BGA substrate 10. The BGA substrate 10 is coated with a film of epoxy 15. Subsequently, a chip 20, which has two arrays of bonding pads 25 on the chip center, is adhered to the BGA substrate 10 with the epoxy 15. Finally, gold bonding wires 35 are bonded on the arrays of bonding pads 25 on the chip center and are also bonded on gold fingers 40 of the BGA substrate 10. As the front view, the bonding wire 35 is excessively long for connecting the bonding pads 25 to the gold finger 40. For example, it is available that the maximum length of wire bond is 140 mil (one-thousandth of an inch) on the chip with 10.526 mm L1×4.984 mm W1 and total length is 190 mil. However, the length of gold bonding wire gets up to 198 mil on the die, and total gets up to 218 mil if using above packaging process. When the chip is poured liquid encapsulated epoxy into a mold, it assumes the risk of reducing yield. Particularly, when the pad pitch is too dense, e.g. 50-100 μm, the wire collapsing happens to lead to short because the gold bonding wire is extruded by the liquid encapsulated epoxy. FIG. 1A shows that pad pitch is 90 μm, and the interval between two arrays of bonding pads is 0.425 mm. The typically defective rate is 50% to 80%.
FIG. 2 is a WBGA (Window BGA) package. The BGA substrate 10 has a window on its center. The chip 20 is connected to bonding pad 25 and to gold finger 40 of BGA substrate with gold bonding wire 35 by Flip-Chip package. Although it can reduce the length of gold bonding wire 35, the cost is increased. Another disadvantage is that WBGA is not suited to some package specifications.
The object of the invention is to provide a technology about the dummy wafer redistribution layer to reduce cost of package.